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Description
As traditionally viewed, the development of contact metamorphic aureoles is an isochemical and constant-volume process. However, preliminary research conducted at San Diego State University during 1994-1996 indicated that volume loss associated with the transfer of elemental mass out of contact metamorphic aureoles may occur, particularly under the conditions of relatively high fluid-to-rock ratios. In this thesis, the specific question that I am concerned with is during crystallization and development of the aureole of the Emigrant Gap composite pluton, was there movement of elements out of the aureole under the conditions of high fluid-to-rock ratios, and as a result a concomitant loss of volume? Therefore, I completed a statistical and chemical study of samples collected from outside and inside a well defined and mapped portion of the contact metamorphic aureole of the Middle Jurassic Emigrant Gap composite pluton, northern Sierra Nevada, California. Though no statistically significant change in mass occurred in 40 out of 41 elements analyzed during this study, the one element that does exhibit a statistically significant change in mass is silica, which also makes up the greatest bulk of the rock mass in the argillites of the aureole (~60%). Moreover, the loss of silica mass translates into a statistically significant change in argillite volume during aureole development (~11%). The selective mobilization of silica is probably a function of its relatively high solubility in fluids that were flushed through the aureole. In fact, numerical modeling and calculations presented indicate that relatively high fluid to rock ratios are needed to produce the observed change in silica mass (~100 to 1). In contrast, similar procedures indicate that unusually high fluid to rock ratios would be needed to change the mass of titanium (~105-108), a result that is consistent with the idea that titanium was relatively immobile during contact metamorphism. However, the composition of the infiltrating fluid is unknown, and as a result the fluid to rock ratios cited in this thesis should be considered as a first order approximation to a considerably more complex problem. The results of work presented in this thesis suggest that part of the room making process for the Emigrant Gap composite pluton was the dissolution and removal of silica from the aureole. This loss of volume is thus another potential near-field mechanism for producing space during pluton emplacement. These results then indicate that the traditional view of contact metamorphism as an isochemical and constant-volume process may indeed be overly simplistic and in need of reevaluation.
